Polydienes may be produced by solution polymerization, wherein conjugated diene monomer is polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and products, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent. Nevertheless, the presence of solvent presents a number of difficulties. The solvent must be separated from the polymer and then recycled for reuse or otherwise disposed of as waste. The cost of recovering and recycling the solvent adds greatly to the cost of the polymer being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polydienes may also be produced by bulk polymerization (also called mass polymerization), wherein conjugated diene monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced.
Despite its many advantages, bulk polymerization requires very careful temperature control, and there is also the need for strong and elaborate stirring equipment since the viscosity of the polymerization mixture can become very high. In the absence of added diluent, the high cement viscosity and exotherm effects can make temperature control very difficult. Consequently, local hot spots may occur, resulting in degradation, gelation, and/or discoloration of the polymer product. In the extreme case, uncontrolled acceleration of the polymerization rate can lead to disastrous “runaway” reactions. To facilitate the temperature control during bulk polymerization, it is desirable that a catalyst gives a reaction rate that is sufficiently fast for economic reasons but is slow enough to allow for the removal of the heat from the polymerization exotherm in order to ensure the process safety.
A technologically useful bulk polymerization process for the production of polydienes is disclosed in U.S. Pat. No. 7,351,776. According to this patent, a multi-stage continuous process is employed wherein polydienes are polymerized within a first step in the substantial absence of an organic solvent or diluent. The polymerization medium is then removed from the reaction vessel and transferred to a second vessel wherein the polymerization reaction is terminated. This termination occurs prior to a significant monomer conversion. Termination may include the addition of a quenching agent, a coupling agent, a functionalized terminator, or a combination thereof. Following termination, the polymerization medium is then devolatilized.
Within the production of polydienes, such as those produced by the bulk polymerization processes described in U.S. Pat. No. 7,351,776, several functionalizing agents and/or coupling agents have been found to be particularly advantageous. For example, U.S. Pat. No. 8,314,189 teaches that functionalized polymers can be prepared by reacting a reactive polymer with a heterocyclic nitrile compound. These reactive polymers can advantageously be prepared using bulk polymerization processes in a lanthanide-based catalyst system. The resultant functionalized polymers exhibit advantageous cold-flow resistance and provide tire components that exhibit advantageously low hysteresis.
In the art of manufacturing tires, it is desirable to employ rubber vulcanizates that demonstrate reduced hysteresis, i.e., less loss of mechanical energy to heat. For example, rubber vulcanizates that show reduced hysteresis are advantageously employed in tire components, such as sidewalls and treads, to yield tires having desirably low rolling resistance. The hysteresis of a rubber vulcanizate is often attributed to the free polymer chain ends within the crosslinked rubber network, as well as the dissociation of filler agglomerates. Functionalized polymers have been employed to reduce hysteresis of rubber vulcanizates. The functional group of the functionalized polymer may reduce the number of free polymer chain ends via interaction with filler particles. Also, the functional group may reduce filler agglomeration, Nevertheless, whether a particular functional group imparted to a polymer can reduce hysteresis is often unpredictable.